Some electronic equipments having a display device, such as a mobile phone or a portable computer, are provided with a function to change a brightness of a display part in response to a surrounding light. For example, there is developed a mobile phone having a display part of which brightness is controlled automatically in response to a surrounding light. That is, a brightness of a display part is controlled so that a brightness is increased in a bright place to cause a display on a display part to be easily recognized and a brightness is decreased in a dark place to prevent an excessive brightness. There is a case in which a function to automatically turn on a back light when an atmosphere gets dark in order to cause operation keys to be visible by illuminating the operation keys from a backside when operating a mobile phone in a dark place.
In order to perform a control to automatically increase a brightness of a display part or automatically turn on a back light of operation keys, it is necessary to detect a surrounding light. Thus, an illuminance sensor, which detects a luminance, is incorporated into a mobile phone. A control part of the mobile phone controls a brightness of a display part or lighting of a back light of operation keys in accordance with a signal from the illuminance sensor.
In the case of a mobile phone, such an illuminance sensor is mounted to a board provided in the housing of the mobile phone in many cases. In the case where the illuminance sensor is mounted in the board provided in the housing of the mobile phone, it is necessary to pull in a surrounding light to the illuminance sensor from outside the housing of the mobile phone. Accordingly, a lighting window is provided to the housing of the mobile phone in order to take a light into the housing.
The above-mentioned lighting window is provided in the vicinity of a display part of a mobile phone in many cases. The lighting window is for leading a light incident thereon to an illuminance sensor inside the mobile phone. Thus, the illuminance sensor is preferably located directly under the lighting window. However, a display device such as a liquid crystal display device (LCD) is located between a board to which the illuminance sensor is mounted and a frame constituting the display part in many cases. In such a case, a distance between the lighting window and the illuminance sensor must be larger than the thickness of the liquid crystal display device (LCD). Thus, a light-blocking part, which blocks a space between the lighting window and the illuminance sensor so that a light other than an incident light through the lighting window does not enter through the space between the lighting window and the illuminance sensor.
FIG. 1 is a schematic illustration indicating a positional relationship between the lighting window, the illuminance sensor and the light-blocking part which blocks the space therebetween. As illustrated in FIG. 1, the space from the lighting window 1 to the illuminance sensor 2 is surrounded by the light-blocking part 3, and, thereby, no surrounding light (for example, a leaking light from a display device near the illuminance sensor 2) is incident on the illuminance sensor 2 because the surrounding light is blocked by the light-blocking part 3. Thus, by providing the light-blocking part 3 between the lighting window 1 and the illuminance sensor 2, a light traveling through parts other than the lighting window 1 is blocked so that the illuminance sensor 2 can detect an illuminance of the surrounding area with high accuracy.
It is suggested, as a structure for leading a light to a sensor, to cause a light exiting from an optical fiber to incident on a light-receiving element (sensor) after being passed through a tapered hole (for example, refer to Patent Document 1). This suggestion is to cause a light, which exits an optical fiber and has a small beam diameter, to incident on an entire surface of a sensor while being passed through a gradually enlarged tapered hole.
Patent Document 1: Japanese Laid-Open Patent Application No. 01-113709
Although a light exiting perpendicularly from the lighting window 1 is directly incident on the illuminance sensor 2 if the light-blocking par 3 as illustrated in FIG. 1 is provided, a light exiting obliquely from the lighting window 1 is incident on the illuminance sensor 2 after being reflected by an inner surface of the light-blocking part 3. Here, the light-blocking part 3 is integrally formed as a part of a housing or a frame of a mobile phone in many cases. In such a case, the light-blocking part 3 is in the same color as the housing or the frame of the mobile phone. For example, a black mobile phone has a black inner surface (a surface reflecting a light) of the light-blocking part 3, and a white mobile phone has a white inner surface (a surface reflecting a light) of the light-blocking part 3.
A reflectance differs from black to while, and a reflectance of white is larger than a reflectance of black. Accordingly, an amount of light incident on the illuminance sensor 2, when the white light-blocking part 3 is provided, is larger than an amount of light incident on the illuminance sensor 2 when the black light-blocking sensor 3 is provided. That is, if the same amount of light enters through the lighting window 1, an illuminance detected by the illuminance sensor 2 when the white light-blocking part 3 is provided is larger than an illuminance detected by the illuminance sensor 2 when the black light-blocking part 3 is provided, because an attenuation of light due to a reflection by the white light-blocking part 3 is smaller than an attenuation of light due to reflection by the black light-blocking part 3. As mentioned above, an illuminance detected by the illuminance sensor 2 varies depending on the color of a housing or the frame of a mobile phone, which generates variations in controlling a brightness of a display part and controlling in switching a back light of operation keys.
Thus, it is desirous to realize a light guide mechanism for the illumination sensor, which enables a detection of a luminance without influences of the color of the light-blocking part.